A thermopile sensor is a two terminal composite device that is sensitive to thermal or infrared (IR) energy and is characterized by an anode (positive terminal) and a cathode (negative terminal) terminating multiple thermocouple devices in series. A thermopile sensor typically outputs a diminutive voltage, proportional to the energy of an impinging IR wave, at its anode with respect to the voltage present at the cathode. The thermopile sensor output voltage amplitude typically ranges from nanovolts to a few millivolts depending on the amount of absorbed IR energy, the optical filter (if any), the material the thermocouples are made of, and on how many thermocouples make up the pile.
Low noise, chopper stabilized amplifiers with high gains are often employed to amplify the tiny thermopile voltages before being digitized with an analog-to-digital converter (ADC). The ADC can also have a resolution where its least significant bit (LSB) value is lower than either the signal or the amplified voltage being measured. The readout circuit may select the amplifier gain and ADC resolution in such a way as to optimize power and bandwidth. Regardless of the ADC resolution, some form of buffering is usually required because thermopiles have non-zero output resistance, which can exceed 10 kOhm. High thermopile output resistance generally requires low pass filtering in the gain path up to the ADC in order to reduce the associated Johnson noise of the thermopile.
A problem emerges when one tries to resolve small signal dependent changes or differences in the thermopile output voltage(s) when the nominal or time averaged thermopile voltage already requires high gain to resolve. The small changes or differences in thermopile output voltage(s) between two instants in time due to variations in the detected IR may be orders of magnitude less than the nominal thermopile output voltage, and thus, difficult to visualize without digital signal processing (e.g. averaging, wavelet filtering, etc.).
However, digital signal processing requires more samples to be acquired, adds cost in terms of memory and computation hardware and thus increases the power dissipation of a readout. Thus, an increase in the analog resolution of the measurement by one or more orders of magnitude may reduce the power dissipation, processing time and cost in the overall readout circuit and associated signal processing.
A classic solution to the problem of resolving small analog difference signals is to either increase the gain in the path leading to the ADC and/or increase the resolution of the ADC. However, high gain increases the risk of saturating the outputs of the amplifier chain(s) and/or over-ranging the ADC(s) when some or all of these components reside on a low voltage readout integrated circuit (ROIC). For example, let's assume that the nominal output voltage of a thermopile exposed to a steady stream of equal amplitude IR packets is 1 mV. This would require a gain of 1,000 to amplify it to 1.0V. If a subsequent IR packet impinging on the thermopile generates, for example, a +1 uV output voltage deviation, the thermopile output signal increases by 1 uV to 1.001 mV, and with the same gain of 1,000 results in an amplified signal of 1.001V. If the ADC range/resolution is 2V/12-bits (for example), then the LSB is 488 uV and said +1 uV deviation would be ultimately resolved as a +2 LSB delta. This may not be large enough to discern from the system noise floor without substantial digital signal processing.
A large ADC output deviation count in response to any IR signal change is usually needed, for example, at least 10 LSBs in order for the signal change to be discerned from the noise floor. In the above example, to achieve the required 5X higher resolution for the 1 uV change, a gain of 5,000 would be needed but this would in turn require a single-ended amplifier to try to output a voltage of 5.005V. This would severely saturate the amplifier and ADC in any low voltage CMOS process node.
The problem is particularly acute when there are multiple thermopile “pixels” such as in a linear or rectangular thermopile array in an IR focal plane. The pixel analog output voltages can all be slightly different from one another depending on their position in the array, which is exposed to the IR radiation generating a plurality of tiny voltage differences. The voltage differences may be orders of magnitude less than the average value of the ensemble of the elements. This would influence the measured resolution of the image along or across the array, which would also affect the image quality of an IR focal plane array.
Higher ADC resolution also may be used to increase the sensitivity of the measurement but with higher cost and power dissipation due to the increased complexity of the ADC architecture.
Thus, a method to increase the sensitivity of a readout channel with a reduced risk of over-ranging or saturating the amplifier(s) or ADC(s) could reduce the cost, power dissipation and processing time of low-level analog signals.
Further, a method to increase the sensitivity of a readout channel without increasing the resolution of the digitizer could also reduce the cost, power dissipation and processing time of low-level analog signals.
In particular, a method to increase the sensitivity of a readout channel by applying a singular voltage adjustment to a plurality of thermopile amplifiers could also reduce the cost, power dissipation and processing time of low-level analog signals.
Further, a method to increase the sensitivity of a readout channel in a low voltage sub-micron CMOS process node could increase the number of thermopile sensors that could be discerned with increased resolution.